Added support for immutablejs

[rt2zz]

@actra-gschuster thanks for contributing, however we cannot reasonably expect all users to have or add immutablejs. It looks like the only way to reasonably achieve this is to make a separate transform with immutable support. Following the naming convention of the ecosystem you might call the modules something like redux-persist-transform-filter-immutable. Long but explicit :)

[actra-gschuster]

Using immutablejs is optional, it still supports plain objects. But nevermind, I'll set up a repo and publish it there.

[rt2zz]

immutable objects is optional, but with this PR having the dependency is required

[edy]

closed. see above comments and https://github.com/actra-development/redux-persist-transform-filter-immutable